The face stability of shield tunnel is studied using the upper bound limit analysis method.An improved collapse mechanism is proposed for the stability analysis of tunnels in pure cohesive clay under undrained conditions and the corresponding formulas are deduced.The computer program is coded to calculate the optimal upper bound for support pressure.The validity of the improved collapse mechanism for the stability of TBM excavation in clays and its solution are demonstrated by comparison with the existing limit analysis solutions and the finite element limit analysis solutions.The least support pressure is expressed by the summation of the cohesion c

surcharge load q and gravityγby the corresponding influence factors Nc

Nqand Nγ.And the expressions of the influence factors are also obtained and discussed.